There exists a multitude of press machines that are presently in widespread use for compacting powdered materials into solid or semisolid form by exerting force on at least one set of two opposing punches or pistons entering once or twice a plurality of dies or pressing matrices containing the material to be compacted (e.g. U.S. Pat. Nos. 4,408,975 to Hack, 4,569,650 to Kramer, 4,680,158 to Hinzpefer et al., 4,880,373 to Balog et al., 5,017,122 to Staniforth, 5,116,214 to Korsch et al., 5,148,740 to Arndt et al., 5,202,067 to Solazzi et al., 5,211,964 to Prytherch et al., 5,462,427 to Kramer and 5,607,704 to Schlierenkamper et al.). A number of inventions relate to press machine instrumentation (e.g. 3,255,716 to Knoechel et al., 4,016,744, 4,030,868 and 4,099,239 to Williams, 4,100,598 to Stiel et al., 5,229,044 to Shimada et al.) and control (e.g. 3,734,663 to Holm, 4,121,289 to Stiel, 4,570,229 to Breen et al., 4,817,006 to Lewis, 5,288,440 to Katagiri et al., 5,491,647 to O'Brien et al.).
The examples of applications using press machines include pharmaceutical tablets and caplets, coal briquettes, ammunition, nuclear pellets, metal and plastic machine parts, ceramic isolators, catalysts or ferments, briquettes for X-ray spectrochemical analysis, grain pellets, coins, and so on.
In a compaction process, the mechanical and other properties of the compound are influenced primarily by powder composition, as well as by speed, movement profile and the force of punches that are in contact with the powder under compression. In a typical production environment, compressed products are usually made in large quantities at fast speeds. During a product development stage and for process troubleshooting, smaller quantities of the powder are often available while the press machines may be much slower and, in general, quite different from those used in production. For a product and process optimization, therefore, it is desirable to be able to reproduce typical production conditions to avoid problems in the scale-up of processing factors.
In the prior art, compaction simulators based on hydraulic actuators are used for the purpose of mimicking the compaction profile of different press machines. Typically, a pump pushes pressurized oil to the cylinder units that, in turn, move the punch holders with the help of valves and hydraulic tanks. The movement of the two punches entering the die cavity with the material to be compacted can be controlled by the actuators in order to follow any prescribed path. The path is specified in the form of a geometrical function (such as a sinusoid or a tooth-saw waveform) or may have any arbitrary form as recorded during a compaction event on another press machine with the aim of mimicking this event on the simulator. The recording from another press machine may contain either the punch displacement path or the force change profile. The desired path, whether theoretical or empirical, is further digitized by a computer, and a series of discrete commands are then given to the hydraulic actuators that are diligently reproducing either the movement of the punches or the force curve (see, e.g. U.S. Pat. No. 5,517,871 to Pento).
There are several problems with the existing compaction simulators that render them practically useless for process scale-up:
The hydraulic actuators can follow any prescribed path but theoretical paths such as a sinusoid are not representing the punch movements in the production press. Fixed geometry of the functions used to produce theoretical waveforms do not take into account the compressibility of the powders under compression (for force curve simulation) or the mechanical deformation of the punches and press assembly (for punch displacement simulation).
The empirical waveform that can be obtained from a production press depends on the brand and model of the press, the shape and size of the tooling, the production rate and the viscoelastic stress/strain behavior of the powder being compressed. Since the composition of optimized powder is unknown during the development stage, the present art solution is to use powder "similar" to the one being developed even though the degree of similarity can never be quantified or made sufficient for quantitative analysis of the compatibility. In addition, the multitude of possible values of such factors as tooling, press speed and geometry make this empirical approach to compaction simulation highly impractical.
In the currently available compaction simulators, the motion of punches is controlled by hydraulic actuators that periodically compare the current position or the force of the punches with the digitized prescription. Such comparison and the subsequent correction can not be made with sufficient frequency to assure smooth trajectory without jerking or tooth-saw like movement, even with the fastest reported data acquisition and control rate of 5 kHz per channel.
In the currently available compaction simulators, there is a choice of simulating either the motion of the punches (punch penetration curves) or the force/time path (compression profile). It is impossible to mimic both.
As a result, there is a wide discrepancy between the resulting properties of compounds obtained from different simulators following the same prescribed path for the same compound. The reported difference of 10 to 16 percent have been attributed to elastic distortion and loading characteristics of the hydraulic systems.